When tax day rolls around on April 15, nearly half of Americans will have paid no federal income tax – because they don't legally owe any. Millions of others will pay a tax – but at rates that can vary widely based on circumstances.

Is this fair?

It's a debate that goes back as far as the income tax itself. But the issue is a hot one now as the nation is transitioning from tax policies of former President Bush to those of President Obama. It's a transition from an era of tax-rate cuts for all income brackets to a time of rate hikes for high-earning households alongside broad new credits and rebates intended to boost a still-weak economy.

Whether you favor a flat tax, the "fair tax," a soak-the-rich tax, or no tax (where can we get that deal?), here's some food for thought – facts about who pays what in taxes:

• The roughly 45 percent of Americans who owe no income tax are heavily weighted in certain groups based on income and family status, according to the The Tax Policy Center, the nonpartisan research group that has run the numbers. More than half the tax-return filers in each of these groups owe no taxes: Those who earn less than $30,000, those who are elderly, and those who are single with children.

• The current US tax system is progressive, with higher-income people tending to shell out more of their income in taxes than lower-income filers. According to Internal Revenue Service data, filers with $35,000 in taxable income paid about 10 percent of that income in taxes on average, while filers at $200,000 in taxable income paid 24 percent of their income to the IRS in 2007.

• Not owing federal income tax is not the same as not paying any taxes at all. Workers will fork over payroll taxes for the Social Security and Medicare programs, and various sales taxes, including the federal gasoline tax.

• What you owe depends on a lot more than just how much income you make. Among about 650,000 American tax filers with $500,000 to $1 million in income, 1,646 were categorized by the IRS as "nontaxable" returns in 2007, for example, meaning they paid no income tax. Chalk it up to the way Congress wrote the rules, and the way accountants learn to navigate those rules.